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bequerel 23rd May 2007 01:33 PM

C3m linestage
I am thinking about making a parafeed C3m linestage.
However I have not been able to find typical values for this tube when triode strapped.

I am especially looking for the plate resistance (Rp) value in triode mode.

Several people have used the Lundahl 1676 transformer for prafeed linestages.
Any comments regarding this transformer in combination with the C3m?

kstagger 23rd May 2007 01:36 PM

I made an RC coupled triode-connected (whew!) C3m linestage - it sounds very neutral.

a warning about the C3m - I've had two C3ms die because I moved them while they were warm. Not hot - just warm after 3-5 minutes of use while I was doing some troubleshooting. So make sure you let the tubes cool down before you unplug 'em or turn over your preamp. Also - every C3m I've ever used (I have 5 pairs of different brands) have been very microphonic.

bequerel 23rd May 2007 01:43 PM

Would you care to share which operation points you used?
Do you use the filaments on AC or DC?

(I am fishing for the plate resistance in triode mode to be able to find a suitable transformer.)

jane 23rd May 2007 02:24 PM

You'll find the triode plate curves here:
They will tell you rp in your operating point. :)

Jan E Veiset

bequerel 23rd May 2007 02:39 PM

Thank you, "jane".:)

Following your projects at with great interest!,11359.0.html

Klimon 23rd May 2007 03:18 PM


(I have 5 pairs of different brands)
Siemens, Lorenz, TFK, Valvo....

So far I've seen (dismantling Siemens and TFK) two varieties of internal constructions; Siemens with mesh and TFK solid (think it's the second screen).... Also read somewhere that Siemens is the only manufacturer of C3m, so both could be Siemens variations over time... Dunno what's true.


bequerel 23rd May 2007 03:24 PM

Cool tube!
Have a look at the guts of a Siemens C3m. A small mesh pentode/triode inside!

AndreasS 23rd May 2007 03:53 PM

The mesh is a screen.


kstagger 23rd May 2007 03:57 PM

2 Attachment(s)
I have Philips, Haltron, Valvo, Siemens and Lorenz C3ms... I would say that the majority of them look to be Siemens made... vintage varied with the Haltron looking the oldest to me. Didn't know that the TFK was had a solid-mesh - interesting. Some of the older EF40s used a solid-mesh and it's also used with some EF86 tubes.

I used b+ 350V
15k plate
430 ohm cathode with bypass

it's DC coupled to a 5687 white cathode follower...

off the top of my head - bias was ~6V

I used DC on the filaments... it was easier for me run a 7820 regulator with transformer I had on hand then buy a 20V unit.

bequerel 23rd May 2007 04:36 PM

Great looking preamp, kstagger! :)

Nice to see projects where the constructors have made an effort to make their projects look good, too.

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